The invention relates to a rib waveguide device with a mode filter section and particularly to a silicon rib waveguide device.
Silicon rib waveguide devices may conduct light in single mode or multimode. The dimensions of the rib are important parameters determining the mode of light transmission. The vertical depth of the rib is such that the transmission is normally single moded in the vertical direction and references in this application to variation between single and multimode transmission relate to variations across the width of the rib.
When constructing waveguide devices on a single integrated silicon chip, some optically transmissive paths may be multimode across the transmission path and others may be single moded. Junctions between multimoded and single moded transmission paths may be required. Furthermore, rib waveguides may need to have straight sections or curved sections. Circuit components on the chip may themselves incorporate curved waveguides such as multiplexing and demultiplexing array waveguides and other parts of the chip may require curved waveguides in order to form compact interconnections between components on the chip. The radius of curvature that can be accepted for a curved waveguide will depend on the rib waveguide width as light losses from the waveguide will vary with radius of curvature. It has been found for example that with a 4 xcexcm wide silicon rib waveguide the minimum radius of curvature without unacceptable losses is 20 mm. However if the waveguide has an increased width to 6 xcexcm then the radius of curvature may be reduced to 12 mm with approximately the same light loss in passing through the curved section. In order to make compact devices it is desirable to be able to reduce the radius of curvature of curved sections but the use of greater width for the waveguides means using a waveguide width which in a straight section would be multimoded across its width. On passing around a curved waveguide any higher modes above the fundamental would suffer some loss from scattering on passing around the curve but some may remain when the curved waveguide joins a serially connected straight waveguide section.
It is an object of the present invention to provide a mode filter for use in combination with curved and straight silicon rib waveguides so that multimode propagation is filtered out by a narrower straight waveguide rib following a wider curved waveguide rib.
The invention provides a silicon rib waveguide device including a mode filter section serially connected to a curved rib waveguide section, said curved waveguide section having a first rib width large enough to support multimode transmission, said filter section having a straight waveguide section with a second rib width smaller than said first rib width and supporting only single mode transmission, said filter section having a first taper section connecting the curved waveguide section to the straight waveguide section and tapering in towards one end of said straight section, and a second taper section connected to an opposite end of the straight section and tapering outwardly from the straight section for connection to an optical transmission device having an optical path width greater than said second rib width.
The waveguide device may include a plurality of curved rib waveguides formed on a common silicon planar device, each connected to a similar respective mode filter section.
The first rib width may be at least 50% greater than said second rib width.
The straight section may have a length between the taper regions of at least 200 times the said second rib width.
The length of each taper section may be at least 80 times the said second rib width.
The second rib width may be approximately 4 xcexcm and the first rib width approximately 6 xcexcm.
The radius of curvature of the curved section may be less than 20 mm.
The radius of curvature of the curved section may be approximately 12 mm.
Said second taper section may be connected to a multimode interference (MMI) device.
The mode filter section may be optically connected with a multiplexer or demultiplexer device using a curved array of waveguides.
The waveguide device is preferably formed as an integrated silicon chip device.